Although Al batteries having a high energy density are currently expected to be used practically, they have only been realized for primary batteries using an aqueous solution for the electrolyte and some air batteries (see, for example, “Handbook of Batteries”, Asakura Publishing Co., Ltd., p. 687, 1996).
As is disclosed in the “Handbook of Batteries”, Asakura Publishing Co., Ltd., p. 687, 1996, aluminum air batteries have a high oxidation-reduction potential and hydrogen is generated as a result of electrolysis of the electrolyte in the form of water taking place on the anode. In addition, since hydroxide ions (OH−) are used for the mobile ions, aluminum hydroxide is formed at the anode and since this is difficult to dissolve in water, it creates the problem of inhibiting discharge to the cathode. These problems are caused by the use of an aqueous electrolyte and the use of OH− ions for the mobile ions.
Several proposals have been made for realizing aluminum batteries by using ions other than OH− ions for the mobile ions through the use of a non-aqueous electrolyte. The Al salt, AlCl3, in particular has long been known to comprise a room temperature molten salt and quaternary ammonium, and aluminum batteries have been disclosed that use this as a non-aqueous electrolyte (see, for example, Japanese Patent No. 2977252, Japanese Patent No. 3380930 and Japanese Patent Application Publication No. H3-238769).
Moreover, a non-aqueous electrolyte has also been disclosed that uses Al3+ ions as mobile ions by using an Al salt other than AlCl3 (see, for example, Japanese Patent Application Publication No. H6-52898).
In addition, batteries have been disclosed that have a non-aqueous electrolyte that uses lithium ions for the mobile ions, and use a carbon material for the material that occludes and releases aluminum ions (see, for example, Japanese Patent Application Publication No. 2003-163030 and Japanese Patent Application Publication No. H11-297355).
On the other hand, solid electrolytes have been disclosed that use ions of a trivalent metal, a specific example of which is Al2(WO4)3, for the mobile ions. Since these electrolytes allow high-density charge transfer, they have been disclosed with respect to application to the electrolytes of some gas sensors and solid secondary batteries (see, for example, Japanese Patent Application Publication No. 2005-149982, Japanese Patent Application Publication No. H11-203935 and Japanese Patent Application Publication No. H10-255822).
Materials having the same crystal structure as Al2(WO4)3 and so on are known to be materials having low thermal expansion (see, for example, Japanese Patent Application Publication No. 2003-89572).
Electrode active materials for non-aqueous electrolyte secondary batteries are known that are composed of compounds represented by the formula M2(MoO4)3 (wherein, M is at least one type of element selected from the group of elements included in group 13 of the periodic table such as Al) (see, for example, Japanese Patent Application Publication No. 2001-85011).
Moreover, compounds have also been disclosed that have a tungsten-bronze structure represented by AlxWO3 as typical compound oxides of Al, W and Mo (see, for example, Mat. Res. Bull., Vol. 2, pp. 809-817 (1967) and C.R. Acad. Sc. Paris, t.266, serie C, pp. 1066-1068 (1968)).